1. Field of the Invention
The present invention relates an optical system that is suitable for an optical apparatus, for example, a silver salt film camera, a digital still camera, a video camera, a digital video camera, a telescope, a binocular, a projector, or a copier.
2. Description of the Related Art
An optical system that is used in an optical apparatus, such as a digital camera, a video camera, or a projector, is desired to have a wide angle of view and to provide an image without a blur of colors. Also, an optical system that is used in a projector or a single-lens reflex (SLR) camera is desired to have a long back focus.
A retrofocus optical system can easily have a wide angle of view and a long back focus, and hence is frequently used as an image taking optical system or a projection optical system. However, the retrofocus optical system has a refractive power arrangement that is asymmetric about an aperture stop. Lateral chromatic aberration has to be properly corrected.
Typically, in order to correct lateral chromatic aberration in an optical system, a lens made of an anomalous dispersion material is used (see Japanese Patent Laid-Open No. 2001-188172, Japanese Patent Laid-Open No. 2007-178894, U.S. Pat. No. 7,292,398, and U.S. Pat. No. 7,480,102).
However, although the lens made of the anomalous dispersion material is used, it is still difficult to properly correct lateral chromatic aberration that is increased when the angle of view is increased. In particular, to properly correct the lateral chromatic aberration generated because the angle of view is increased, a plurality of lenses that are made of different materials and have predetermined refractive powers have to be arranged at adequate positions in an optical path. If the arrangement is inadequate, it is difficult to properly correct the lateral chromatic aberration when the angle of view is increased.